Move already (i.e. not new) completed files in rtorrent

Ok, just figured this out. Within rtorrent you can open a command line using Ctrl+X. You can do many things from there (I guess this is basic rtorrent management), like printing things (print=$variable=, e.g. print=$d.get_directory=), executing commands (execute=command), or setting variables (variable=newvalue).

From this prompt you can move the finished torrent elsewhere, BUT note that it is neither necessary nor sufficient (see below). For instance, using the example from the .rtorrent.rc file given in the original question :

execute=mv,-u,$d.get_base_path=,$d.get_custom1=

However, this command will prevent rtorrent from continuing to seed the torrent, which is why it is not sufficient. In order to continue seeding, you should, still from this command prompt, set the download directory for this torrent to the new location :

d.set_directory=/path/to/new/directory/

Finally, the execute command is not necessary : you can move the torrent the way you want (i.e. outside rtorrent), as long as you set the new directory as explained above.

After that it may be necessary to reopen the torrent (if it's marked as [CLOSED]) using Ctrl+R.